Touch panels have recently become widely adopted as the input device for high-end portable electronic products such as smart-phones and tablet devices. Although, a number of different technologies can be used to create these touch panels, capacitive systems have proven to be the most popular due to their accuracy, durability and ability to detect touch input events with little or no activation force.
The most basic method of capacitive sensing for touch panels is the surface capacitive method—also known as self-capacitance—for example as disclosed in U.S. Pat. No. 4,293,734 (Pepper, Oct. 6, 1981). A typical implementation of a surface capacitance type touch panel is illustrated in FIG. 1 and comprises a transparent substrate 10, the surface of which is coated with a conductive material that forms a sensing electrode 11. One or more voltage sources 12 are connected to the sensing electrode, for example at each corner, and are used to generate an electrostatic field above the substrate. When an input object 13 that is electrically conductive—such as a human finger—comes into close proximity to the sensing electrode, a capacitor 14 is dynamically formed between the sensing electrode 11 and the input object 13 and this field is disturbed. The capacitor 14 causes a change in the amount of current drawn from the voltage sources 12 wherein the magnitude of current change is related to the distance between the finger location and the point at which the voltage source is connected to the sensing electrode. Current sensors 15 are provided to measure the current drawn from each voltage source 12 and the location of the touch input event is calculated by comparing the magnitude of the current measured at each source. Although simple in construction and operation, surface capacitive type touch panels are unable to detect multiple simultaneous touch input events as occurs when, for example, two or more fingers are in contact with the touch panel.
Another well-known method of capacitive sensing applied to touch panels is the projected capacitive method—also known as mutual capacitance. In this method, as shown in FIG. 2, a drive electrode 20 and sense electrode 21 are formed on a transparent substrate (not shown). A changing voltage or excitation signal is applied to the drive electrode 20 from a voltage source 22. A signal is then generated on the adjacent sense electrode 21 by means of capacitive coupling via the mutual coupling capacitor 23 formed between the drive electrode 20 and sense electrode 21. A current measurement means 24 is connected to the sense electrode 21 and provides a measurement of the size of the mutual coupling capacitor 23. When the input object 13 is brought to close proximity to both electrodes, it forms a first dynamic capacitor 27 to the drive electrode 20 and a second dynamic capacitor 28 to the sense electrode 21. If the input object is connected to ground, as is the case for example of a human finger connected to a human body, the effect of these dynamically formed capacitances is manifested as a reduction of the amount of capacitive coupling in between the drive and sense electrodes and hence a reduction in the magnitude of the signal measured by the current measurement means 24 attached to the sense electrode 21.
As is well-known and disclosed, for example in U.S. Pat. No. 7,663,607 (Hotelling, Feb. 6, 2010), by arranging a plurality of drive and sense electrodes in a grid to form an electrode array, this projected capacitance sensing method may be used to form a touch panel device. An advantage of the projected capacitance sensing method over the surface capacitance method is that multiple simultaneous touch input events may be detected.
A schematic representation of an exemplary electrode array used in a conventional mutual capacitance touch panel is shown in FIG. 3a. The electrode array includes a plurality of drive electrodes 20 and a plurality of sense electrodes 21 with the mutual capacitors 23 formed at each intersection between any drive electrode and any sense electrode. The drive electrodes 20 are connected to separate voltage sources 22, each of which can supply a voltage excitation signal to the corresponding drive electrode. The sense electrodes are connected to a current measurement means 24, which measures the current generated on each sense electrode by the voltage excitation signal applied to the drive electrodes. The capacitances of all mutual capacitors in the electrode array are measured according to a driving sequence, the timing diagram for which is shown in FIG. 3b. A single frame period (TFRAME) consists of four measurement periods (tS1, tS2, tS3 and tS4). In order to measure the capacitances of the mutual coupling capacitors, the same voltage excitation signal is applied to the drive electrodes one by one, such that during the first measurement period the voltage excitation signal is applied to the first electrode, during the second measurement period the same voltage excitation signal is applied to the second drive electrode, and so on. When the voltage excitation signal is applied to the last electrode of the electrode array, the sequence starts from the beginning to obtain the data for the next frame of operation. The patterns of voltage excitation signals applied during one frame of operation may be represented as a matrix 36, which is an identity matrix in this example.
It is well-known that the accuracy of the estimation of the location of the conductive object may be improved by increasing the signal-to-noise ratio (SNR) associated with the measurement of the capacitance of the mutual coupling capacitors in the array. A known method of increasing the SNR of a system that employs projected capacitance sensing method is disclosed, for example, in US20100060591 (Yousefpor, filed Sep. 10, 2008 and publish Mar. 11, 2010) According to this method, all of the drive electrodes are excited simultaneously during the measurement period. Each drive electrode is supplied with one of two (or more) possible types of signal that differ from each other in amplitude or phase or both. A timing diagram for an alternative driving sequence is shown in FIG. 4. As previously, the capacitances of all mutual capacitors in the electrode array are measured during one frame period (TFRAME) that consists of four measurement periods (tS1, tS2, tS3 and tS4). During one measurement period voltage excitation signals are applied to all drive electrodes of the electrode array and the currents generated in the sense electrodes are measured. The patterns of voltage excitation signals applied to the drive electrodes are different during each measurement period. The patterns of voltage excitation signals applied during one frame of operation may be represented as a matrix 41. A limitation of this method, however, is that the current measurement means 24 may saturate whilst making the capacitance measurements. For example, the capacitance of the mutual capacitors may comprise a portion that changes due to the presence of an object and fixed offset that is insensitive to presence of an input object. The offset signal, also known as the baseline signal may be significantly larger than the maximum change signal. Saturation may arise in the current measurement means due to the presence of the large baseline capacitance signal. As a result, the dynamic range of the sensor may be reduced and the performance of the touch panel may be degraded.
A known solution to the problem described above is to excite the drive electrodes in a pattern corresponding to a maximum length sequence, or M-sequence. Such a system is described in, for example, U.S. Pat. No. 8,730,197 (Hamaguchi, filed Jan. 23, 2012 and issued May 20, 2014). However, since the length of an M-sequence is limited to values that are equal to 2n−1, where n is an integer number, this method is not suitable for touch panels where the number of drive electrodes is not equal to 2n−1.
In a further application of capacitive touch panels, if the sensitivity of the sensor is sufficiently high, objects may be detected at some distance from the sensor substrate. A method of calculating the position and height above surface of input objects is disclosed in U.S. 20140009428 (Coulson, Jan. 9, 2014). The signal-to-noise ratio and dynamic range requirements of such a system may however be more stringent that conventional applications.
A means is therefore sought to operate the touch panel in such a way as to maximize the signal-to-noise ratio of the capacitance measurement without negatively impacting the dynamic range.